Disclaimer: I work at Kradle.

They were never told to lie: one AI is given more information than the others, and the goal of the experiment is to understand how they're gonna leverage that advantage.

Indeed the selfish (optimal?) strategy is to lie, yet some decide to tell the truth anyway. That's why it's an interesting benchmark! More info in the research article: https://kradle.ai/research/four-bridges (released before Fable)